This invention relates to holographically-formed polymer dispersed liquid crystals (H-PDLCs). In particular, the invention relates to multiple grating reflective displays using H-PDLC technology. The invention also relates to H-PDLCs having multiple reflection and transmission gratings.
Polymer dispersed liquid crystals (PDLCs) in their conventional form consist of micrometer-sized liquid crystal (LC) droplets dispersed in a rigid polymer matrix. PDLCs are typically formed using phase-separation or emulsification methods. Photo-polymerization induced phase separation utilizes a mixture of a low molecular weight liquid crystal and a photo-curable monomer. Irradiation of the homogeneous pre-polymer mixture initiates polymerization, which in turn induces a phase separation between the polymer and liquid crystal. The result is a liquid crystal phase separated into droplets and immobilized in a rigid polymer matrix.
FIG. 1A illustrates a conventional PDLC formed by phase separation of a liquid crystal phase from a matrix polymer phase. The entire LC-monomer film is photopolymerized and phase separation occurs randomly throughout the film and results in LC droplets on the order of microns. In the zero-voltage state, the symmetry axis of the droplets is randomly oriented and there is a mismatch of the index of refraction between the matrix polymer and the LC droplets. This condition results in a strongly light scattering (opaque) appearance. By matching the ordinary refractive index of the liquid crystal with that of the matrix polymer, a transparent appearance is achieved when sufficient voltage is applied to reorient the LC droplets. Thus, conventional PDLC displays are capable of switching between an opaque off-state and a transparent on-state, but do not have inherent ability to display color.
Reflective liquid crystal displays have been developed which rely on PDLC materials and, more recently, holographic or optical interference preparative techniques have been used to carry out polymerization to selectively positioned regions of liquid crystal and polymer. Planes of liquid crystal droplets are formed within the sample to modulate the LC droplet density on the order of the wavelength of light. On exposure to an optical interference pattern, typically formed by two coherent lasers, polymerization is initiated in the light fringes. A monomer diffusion gradient is established as the monomer is depleted in the dark fringes, causing migration of liquid crystal to the dark fringes. The result is LC-rich areas where the dark fringes were located and essentially pure polymer regions where the light fringes were located.
The resulting optical interference pattern reflects at the Bragg wavelength, xcex=2nd sin xcex8, where n is the average index of refraction, xcex8 is the angle between the substrate and viewing direction, and d is the Bragg layer spacing. The interference pattern can be selected to form Bragg gratings which can reflect any visible light. In the xe2x80x9coff statexe2x80x9d, that is, with no applied voltage, the LC directors are misaligned and light of the Bragg wavelength is reflected back to the observer. Upon application of an applied voltage, the xe2x80x9con statexe2x80x9d, the device becomes transparent. The reflection intensity is electrically controlled by changing the effective refractive index of the LC droplet planes with an applied voltage. If the refractive index of the LC droplet planes (nLC) is different from that of the polymer planes (np), light of a specific wavelength is reflected by the periodic modulation in the refractive index. If nLC is equal to np, the periodic refractive index modulation disappears and the incident light is transmitted. If the LC has a positive dielectric anisotropy and the ordinary refractive index no is approximately equal to np, the reflection intensity decreases with increasing applied voltage. This results in a material transparent at all wavelengths and all incident light is transmitted.
Displays incorporating these materials have been reported in xe2x80x9cHolographically formed liquid crystal/polymer device for reflective color displaysxe2x80x9d by Tanaka et al. in Journal of the Society for Informational Display (SID), Volume 2, No. 1, 1994, pages 37-40; and also in xe2x80x9cOptimization of Holographic PDLC of Reflective Color Display Applicationsxe2x80x9d in SID ""95 Digest, pages 267-270 (1995). In each of the reported H-PDLC displays, however, reflection gratings capable of reflecting only a single wavelength of light were created. See, Tanaka et al. in U.S. Pat. No. 5,748,272.
A major interest in the display industry is the creation of full color reflective displays. U.S. Pat. No. 5,875,012 to Crawford et al. describes a full-color liquid crystal device incorporating three single-color stacked reflective H-PDLCs, which can be activated alone or in combination to provide a broad spectrum of color. Although this configuration results in high reflection efficiencies, it is complicated to fabricate and requires sophisticated electrical drive schemes.
Date et al. in xe2x80x9cThree-Primary-Color Holographic Polymer Dispersed Liquid Crystal (H-PDLC) Devices for Reflective Displaysxe2x80x9d (Proceedings of the 15th International Display Research Conference, Hamamatsu, Japan, 1995; p. 603) report single exposure films of different color. A red, a green and a blue reflecting H-PDLC are reported formed using a single laser source, in which the different reflection gratings were obtained at different incident angles from different H-PDLC layers. Date also reported the use of prisms to obtain the appropriate cross angles for longer wavelengths of light. Using this technique, a full color reflective display can only be built by stacking three H-PDLC layers that individually reflect at red, green or blue wavelengths. There were no multiple grating films made from a single layer H-PDLC to reflect multiple colors.
There is a need to provide a single layer H-PDLC with multiple reflective gratings for constructing a reflective display device that can have a range of colors. Such displays are desirable due to their simplified configuration and because they are sufficiently reflective at low power and in normal operating environments.
Lastly, multiple Bragg gratings in display panels and other devices are desired because specular reflections off of multiple gratings within the layer would increase the operative viewing angle and improve the quality of the reflected image. There is currently no method which provides such capability in the prior art.
Creating a near infrared reflecting H-PDLC is a difficult task to accomplish due to the large wavelength shift required to create Bragg gratings in the near infrared band (xcx9c1000 nm) using light in the visible range. The use of visible light lasers to fabricate IR H-PDLCs is attractive for a variety of reasons. The beam is visible with the unaided eye which simplifies alignment and fabrication; and IR photoinitiators, needed for the polymer-initiated phase separation of the H-PDLC, are not readily available or are not developed to a point sufficient for use in this application.
Unfortunately, at the incident angles required to form the infrared interference pattern, the glass surface is highly reflective and very little of the light passes through the supporting glass into the LC-monomer layer. Furthermore, some of the light that does enter the layer is in the form of multiple reflections which wash out the interference pattern.
There is a need to provide an infrared reflective modulating device and a method for obtaining an infrared reflective device that addresses the problems and limitations of the prior art.
The present invention provides advances and improvements in the manufacture of H-PDLC compositions. The use of simultaneous, coherent multiple laser beam exposure has been exploited to provide multiple grating liquid crystal devices from a single layer H-PDLC.
In one aspect of the invention, a multicolored reflection liquid crystal display device is provided from a single layer configuration having a pair of substrates having a reflective holographic polymer dispersed liquid crystal (H-PDLC) film disposed therebetween. The H-PDLC film includes liquid crystal and matrix polymer layers which form a reflection grating capable of reflecting a wavelength of light, wherein the H-PDLC film includes at least two different reflection gratings capable of reflecting two different wavelengths of light. The substrate may be made up of ITO-coated glass or plastic.
In preferred embodiments, the H-PDLC comprises three or more different reflection gratings capable of reflecting three different wavelengths of light. The reflection gratings may be superimposed on the same area of the film, or they may be located in specific regions of the film, so as to form holographic elements patterns, i.e., spatial overlap or non-overlap, respectively.
In preferred embodiments, the display may provide holographic elements having non-overlapping reflectance spectra, i.e. spectral non-overlap. The display may be capable of reflecting three primary wavelengths of light, i.e., red, blue and green, cyan-magenta-yellow, or any other combination.
In other preferred embodiments, the reflection gratings are selected to provide a broadband reflection in which the holographic elements have overlapping reflectance spectra. The reflected wavelengths of light are any desired wavelength, and in particular are of the visible energy range and IR energy range.
In other preferred embodiments, the reflected light is of substantially equal intensity. Regions of different reflection gratings may be arranged in an array or may be arranged to produce a preselected pattern.
In another aspect of the invention, a method of making a multicolored reflective liquid crystal display is provided, in which a film comprised of a mixture of a liquid crystal and a photo-polymerizable monomer are simultaneously illuminated with a plurality of holographic light patterns capable of providing liquid crystal layers of different spacings so as to obtain different reflection gratings in each of the regions. A plurality of regions may be illuminated with different holographic light patterns.
In preferred embodiments, the holographic light pattern is obtained by providing at least two pairs of laser light beams, each beam pair incident on the film at a different angle to form an optical interference pattern associated with reflection of a different wavelength of light. Additional beam pairs, e.g., three or more are contemplated.
In another preferred embodiment of the invention, the holographic light pattern is obtained by providing laser light of a different wavelength, each laser light forming an optical interference pattern associated with reflection of a different wavelength of light.
In other preferred embodiments, a mask is placed between each of the laser light beams and the film, and each mask forms a pattern of light and dark regions on the film. Each mask is positioned such that at least one light region of the first beam pair coincides with at least one dark region of the second beam pair within the film, and the film is illuminated whereby photo-polymerization of the monomer takes place and formation of polymer and liquid crystal layers occurs. This gives rise to spatially non-overlapping holographic elements.
In other embodiments, at least two different gratings are introduced into the film in a single illumination step, or the power of the light beams is substantially equal. In other embodiments, the mask is of a grid pattern having alternating transparent and opaque grid squares, or other patterns of transparent and opaque regions. In some embodiments, the grid squares are on the order of about 25 mm2 or less, and preferably about 9 mm2 or less; however, much smaller sizes are contemplated as within the scope of the invention. In addition, shapes other than grid squares may be used, such as, rectangular or circular shapes and the like.
In one embodiment, two beam pairs are used and a two-color display is obtained; and in other embodiments, three beam pairs are used and a three-color display is obtained. The method may provide films having a plurality of spectrally non-overlapping reflectances, such as red, blue and green, or cyan, magenta and yellow. Alternatively, the method may provide films having a plurality of spectrally overlapping reflectances, which gives rise to broadband reflectance.
In yet another aspect of the invention, illumination of the film by a selected beam pair or pairs is blocked by a shutter for a portion of the exposure time of the film. Shuttering may be used to shorten or lengthen the exposure of one beam pair with respect to the other beam pairs.
In another aspect of the invention, an apparatus for preparation of a multicolored reflective liquid crystal display includes means for supporting a film comprised of a mixture of liquid crystal and a photo-polymerizable monomer; a laser source; means for producing at least two pairs of laser light beams, each beam pair capable of directing light onto a film housed in the supporting means at a different angle to form an optical interference pattern within a film associated with reflection of a different wavelength of light; and a mask disposed between each of the laser light beams and the supporting means, each mask forming a pattern of light and dark regions on a film housed in the supporting means and each mask positioned such that at least one light region of the first beam pair coincides with at least one dark region of second beam pair within a film. The apparatus may further include shutters disposed between the laser source and the film for preferentially blocking illumination from one or more beam pairs.
In still another aspect of the invention, a method of making a holographic phase dispersed liquid crystal having multiple gratings, includes providing a film comprised of a mixture of liquid crystal and a photo-polymerizable monomer capable of phase separation of the liquid crystal upon polymerization and having first and second opposing surfaces; and illuminating the film with at least three beams of laser light, wherein at least one beam is incident on the first opposing surface of the film and at least one beam is incident on the second opposing surface of the film, and having at least one region in which three laser beams overlap, whereby upon photo-polymerization of the monomer and phase separation of the liquid crystal/polymer, a transmission grating and two reflection gratings are formed in the three-beam overlapping region. Beam arrangements employing a greater number of beams and resulting in a greater number of gratings also are contemplated.
In a preferred embodiment, the film further has at least one region in which two beams overlap and a reflection grating is formed in the two-beam overlapping region. Beam arrangements employing a greater number of beams and resulting in a greater number of gratings also are contemplated.
The intensity of light incident on the first opposing surface of the film may be unequal or may be approximately equal to the intensity of light incident on the second opposing surface, depending upon the desired outcome.
In other preferred embodiments, the laser beam is incident on the film at a different angle.
In another aspect of the invention, a multiple grating liquid crystal display device includes a pair of substrates having a holographic polymer dispersed liquid crystal (H-PDLC) film disposed therebetween. The H-PDLC film includes a first region comprising liquid crystal and matrix polymer layers forming a transmission grating and a plurality of reflection gratings and at least one second region comprising liquid crystal and matrix polymer layers forming a reflection grating capable of reflecting a preselected wavelength of light. The transmission grating typically exhibits Bragg diffraction (when the spacing between the LC droplet layers is on the order of the wavelength of incident light), but may also exhibit other behavior, such as for example, Raman Nath diffraction (when the spacing between the LC droplet layers is greater than the wavelength of incident light).
It is also within the scope of the invention to combine a plurality of aforementioned single layer H-PDLC films having multiple spectral gratings into a single display device.
xe2x80x9cBragg gratingxe2x80x9d means periodically repeating layers of a polymer and liquid crystal (LC) which form LC planes having a spacing that satisfy the grating equation,                     Λ        =                  λ                      2            ⁢            n            ⁢                          xe2x80x83                        ⁢                          sin              ⁡                              (                                  ψ                  /                  2                                )                                                                        (        1        )            
where xcex is the wavelength of the incident laser light, n is the average index of refraction of the holographic medium, and "psgr" is the angle between the interfering beams. When the light source and the observer are on the same side of the holographic film, the grating is known as a reflection grating. When the light source and the observer are on opposite sides of the holographic film, light is diffracted upon transmission through the holographic film and the grating is known as a transmission grating.
xe2x80x9cHolographic techniquexe2x80x9d, xe2x80x9cholographyxe2x80x9d, xe2x80x9cholographic lightxe2x80x9d, as those terms are used herein refer to the formation of interfering light patterns in a three dimensional space.
xe2x80x9cHolographic elementxe2x80x9d refers to the smallest spectrally distinct element of a display, i.e., the smallest region having a homogenous grating. The holographic element may be defined by one or more electrodes, e.g., multiple gratings may be homogeneously superimposed over a region of the film, however, each of the gratings may switch between on-and off-states at different potentials. In those instances where the holographic element is defined by a single electrode, the holographic element is also a xe2x80x9cpixelxe2x80x9d, i.e., the smallest switchable element of the device.
xe2x80x9cSpatially overlappingxe2x80x9d and xe2x80x9cspatially non-overlappingxe2x80x9d refer to the location of the grating on the film. When the gratings are non-overlapping, a single grating occupies a defined region of the film and does not share the region with other gratings (other than minor and unintentional overlap due to improper alignment).
xe2x80x9cSpectrally overlappingxe2x80x9d and xe2x80x9cspectrally non-overlappingxe2x80x9d refer to the separation between two reflectance peaks. Reflectance peaks are considered non-overlapping if two adjacent spectra do not overlap at full width at half maximum (FWHM).
When referring to spectral reflectance and wavelength, it is understood that the peak wavelength represents the peak centered around a peak maximum. Width of the full peak may vary, but typically is in he range of 20 nm (FWHM) for single grating peaks.